1. Field of the Invention
This invention relates to a focus detecting apparatus by a TTL phase difference detection system suitable for use in a camera or the like, and particularly to a focus detecting apparatus suitable for detecting the in-focus state in a plurality of areas in the predetermined image plane of a photo-taking lens.
2. Related Background Art
FIG. 11 of the accompanying drawings shows the basic construction of a focus detecting apparatus of a TTL phase difference detection system. The focus detecting optical system of this focus detecting apparatus is such that a photo-taking lens 10, a field mask 20, a condenser lens 30, a stop mask 40, a re-imaging lens 50, a photoelectric conversion element array 60, etc. are successively disposed on the optical axis O of the photo-taking lens 10.
A light beam transmitted through the exit pupils 11, 12 of the photo-taking lens 10 forms a primary image of an object near the field mask 20 by the photo-taking lens 10. The extraction of the light beam is effected by the field mask 20 to thereby limit the light beam from the object to the condenser lens 30. The light beam transmitted through this condenser lens 30 is re-imaged as a secondary image on the photoelectric conversion element array 60 by the re-imaging lens 50 via the stop mask 40 which likewise limits any unnecessary light beam.
That is, rearwardly of the primary image formed by the photo-taking lens 10, two secondary images substantially similar to the primary image are re-imaged by the condenser lens 30 and the re-imaging lens 50, and the amount of deviation of the relative position of the two secondary images is detected by the photoelectric conversion element array 60.
Although the various members are disposed so that the re-imaged secondary images may be arranged only in one particular direction, the image of the object by the photo-taking lens 10 may have a luminance distribution in any direction and focus detection becomes impossible for an object which does not have a luminance distribution in a direction fit for the focus detecting optical system. Therefore, a plurality of focus detecting optical systems are disposed in different directions.
Heretofore, in such a focus detecting apparatus, the shapes of a pair of openings corresponding to a pair of re-imaging lenses have been made different for each focus detecting optical system and by the use of a light beam from a larger portion (area) in the plane of the exit pupil, the light beam has been utilized efficiently and it has been intended to realize the compactness of each re-imaging lens.
Referring to FIG. 11, the areas of the openings 41 and 42 in each pair of stop masks 40 for dividing the pupil of the photo-taking lens 10 into two areas are made to differ for each focus detecting optical system. Therefore, the levels of signals output from respective photoelectric conversion element arrays 60 disposed in the respective focus detecting optical systems differ from each other in proportion to the areas of the pair of openings for dividing the pupil of the photo-taking lens 10 in each focus detecting optical system into two areas.
This leads to the problem that when focus detection is effected by the plurality of focus detecting optical systems, a difference in intensity of illumination is created between secondary images re-formed on the pair of photoelectric conversion element arrays provided for each focus detecting optical systems and a difference is created between the levels of the output signals from each pair of photoelectric conversion element arrays.
In order to solve such a problem, it would occur to mind to amplify the output signals from the pair of photoelectric conversion element arrays for each focus detecting optical system at different ratios, but this would cause another problem that signal processing will become complicated.